An oxygen sensor refers to an electronic device that measures the concentration of oxygen in aqueous solutions in the field or in the laboratory. Currently, the most prevalent type of oxygen sensors are electrochemical sensors such as those used to measure oxygen concentration for determining environmental properties of large bodies of water to ensure appropriate environmental conditions for wildlife. These electrochemical sensors use a probe with electrodes to measure oxygen dissolved in a fluid. More specifically, a cathode and an anode are submersed in an electrolyte. Oxygen enters the probe through a permeable membrane by diffusion, and is reduced at the cathode, creating a measurable electrical current. The relationship between the electrical current and the oxygen concentration is a linear one and thus concentration can be determined from the current and calibration settings for the sensor.
There is, however, a need for oxygen sensors in other environments. In particular, there is a need for oxygen sensors in ultra-high purity environments such as those defined by SEMI F57 standard, FDA standards in food and drug processing or the like. This need arises for a variety of reasons. For example, the presence of oxygen in semiconductor manufacturing processing may increase corrosion of materials involved in the process (e.g., copper used in a plating process) among other problems.
Attempts to adapt electromechanical sensors to ultra-high purity environments has proved well-nigh impossible because it is extremely difficult to re-purpose electrochemical sensors that use relatively large metal probes (e.g., stainless steel or aluminum) originally intended as dip probes to sample dissolved oxygen content in rivers, streams, and lakes to ensure minimum oxygen concentrations levels for wildlife to the task of detecting the sub-part per billion (e.g., 1 part in 109) concentrations of oxygen required for use in semiconductor manufacturing processes, food and drug processes, or other ultra-high purity environments. From an industry perspective such attempts have heretofore been failures.
This failure is due in no small part to the very nature of electrochemical sensors. As discussed above, electrochemical sensors use metal probes (or metal housings) that must be inserted in the fluid being measured. The insertion of these metal probe tips, for example in the process fluid of a semiconductor manufacturing process, serves to contaminate the process fluid in which it is inserted, making the use of such electrochemical sensors incompatible with the materials and processes in which they are being utilized and the standards for those processes. As such electrochemical sensors cannot be used with the caustic fluids that may be utilized in these ultra-high purity environments. In other words, there may a fundamental materials incompatibility between the materials of such electro chemical sensors and the fluids used in ultra-high purity environments that prevents adherence to standards that define those environments.
Moreover, the presence of the probe tips of electrochemical sensors in the fluid serves to disrupt the laminar flow of the fluid, agitating the fluid and causing dead legs in the fluid flow path. These disruptions may, in turn, cause unwanted side effects such as bubbles, or variations in dispense rates, etc. that negatively affect the process in which the fluid is being utilized. Other problems with the use of these electrochemical sensors include the fact that electrochemical sensors may have a relatively large form factor and be ill-designed for use in the high pressure flow rates that are sometimes utilized in ultra-high purity environments. Thus, in many cases, such electrochemical sensors may not be used at all in the compact installations sometimes utilized in ultra-high purity environments or may experience a high rate of failure due to leakage or corrosion resulting from poor internal sealing.
What is desired then, are oxygen sensors suitable for use in ultra-high purity environments. In particular, compact oxygen sensors having a small footprint that are also suitable for use in ultra-high purity environments are desired.